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Electric Current

The time rate of flow of charge through any cross-section is called current.
If the flow is uniform, then i = Q/t. Current is a scalar quantity. Its SI unit is ampere (A) and CGS unit is emu and is called biot (Bi), or ab ampere. 1A = (1/10) Bi (ab ampere)
  • An Ampere of current means the flow of 6.25 × 1018 electrons/s through any cross-section of the conductor.
  • The conventional direction of current is taken to be the direction of flow of positive charge, i.e., field and is opposite to the direction of flow of negative charge as shown in Fig. 1.
    Fig. 1
  • The net charge in a current-carrying conductor is zero.
  • For a given conductor, current does not change with change in cross-sectional area. In fig. 2, i1 = i2 = i3.
    Fig. 2

Current due to translatory motion of charge

If n particles each having a charge q, pass through a given area in time t, then i = nq/t.
If n particles each having a charge q pass per second per unit area, the current associated with the cross-sectional area A is i = nqA
If there are n particle per unit volume each having a charge q and moving with velocity v, the current through, cross section A is i = nqvA.

Current due to rotatory motion of charge

If a point charge q is moving in a circle of radius r with speed v (frequency v, angular speed ω and time period T), then corresponding current,

Current carriers

The charged particles whose flow in a definite direction constitutes the electric current are called current carriers. In different situation current carriers are different.
  • Solids: In solid conductors such as metals, current carriers are free electrons.
  • Liquids: In liquids, current carriers are positive and negative ions.
  • Gases: In gases, current carriers are positive ions and free electrons.
  • Semi conductor: In semi conductors, current carriers are holes and free electrons.

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